1. Field of the Invention
The present invention relates generally to system configuration and method for controlling an image projection apparatus. More particularly, this invention relates to an image projection apparatus implemented with coordinated control for turning on and off the light source corresponding to the operation states of the mirror device performing the function as a spatial light modulator.
2. Description of the Related Art
Even though there have been significant advances made in recent years in the technologies of implementing electromechanical mirror devices as spatial light modulators (SLM), there are still limitations and difficulties with displaying high quality images. Specifically, when the display images are digitally controlled, the quality of the images is adversely affected because they are not displayed with a sufficient number of gray scales.
Electromechanical mirror devices have drawn considerable interest because of their application as spatial light modulators (SLM). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several millions for each SLM. Referring to FIG. 1A for a digital video system 1 includes a display screen 2 disclosed in a relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light beams to project illumination for the display images on screen 2. The light 9 projected from the light source is further concentrated and directed toward lens 12 by way of mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate the light 9 into a column of light 8. A spatial light modulator (SLM) 15 selectively redirects a portion of the light from path 7 toward lens 5 to display on screen 2 through data transmitted over data cable bus 18. FIG. 1B shows a SLM 15 that has a surface 16 that includes an array of switchable reflective elements 17, 27, 37, and 47, each of these reflective elements is attached to a hinge 30. When the element 17 is in an ON position, a portion of the light from path 7 is reflected and redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge on the display screen 2, forming an illuminated pixel 3. When the element 17 is in an OFF position, the light is reflected away from the display screen 2, and, hence, pixel 3 is dark.
Each mirror element that constitutes a mirror device to function as a spatial light modulator (SLM) is comprised of a mirror and electrodes. A voltage applied to the electrode(s) generates a coulomb force between the mirror and the electrode(s), thereby making it possible to control and incline the mirror for deflection.
When a mirror is deflected by a voltage applied to the electrode(s), the direction of the reflected incident light also changes. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. When almost all of an incident light is reflected onto a projection path designated for a display image, it is referred to as an “ON light”. When a light is not reflected to the designated projection path for the display image, it is referred to as an “OFF light”.
“Intermediate light” refers to the light reflected to a projection path with a smaller quantity of light than the ON light, and a ratio exists between the incident light reflected to a projection path (i.e., the ON light) and that reflected from a projection path (i.e., the OFF light)
According to the convention of present specification, a clockwise (CW) angle of rotation is positive (+) and a counterclockwise (CCW) angle of rotation is negative (−). A deflection angle is defined as zero degree (0°) when the mirror is in the initial state.
The on-and-off states of the micromirror control scheme as that implemented in the U.S. Pat. No. 5,214,420, and in most conventional display systems, limit the quality of the display. Specifically, applying the conventional configuration of a control circuit limits the gray scale gradations produced in a conventional system (PWM between ON and OFF states), which is limited by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in the conventional systems, there is no way of providing a shorter pulse width than the duration represented by the LSB. The least quantity of light, which determines the gray scale, is the light reflected during the least pulse width. The limited levels of gray scale lead to degradation of the display image.
Specifically, FIG. 1C exemplifies, as related disclosures, a circuit diagram for controlling a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; transistors M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor 9 and a latch 32a, based on a Static Random Access Switch Memory (SRAM) design. All access transistors M9 on Row line receive a DATA signal from Bit-line 31a. The particular memory cell 32 is accessed for writing a bit to the cell by turning on the appropriate row select transistor M9, using the ROW signal functioning as a Word-line. Latch 32a consists of two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states, that include a state 1 when Node A is high and Node B is low, and state 2 when Node A is low and Node B is high.
The mirror is driven by a voltage applied to the landing electrode, and is held at a predetermined deflection angle on the landing electrode. An elastic “landing chip” is formed on the landing electrode, which puts the landing electrode in contact with the mirror, and deflects the mirror toward the opposite direction when the deflection of the mirror is switched. The landing chip has the same potential as the landing electrode so as to prevent a possible short from the contact between the landing electrode and the mirror.
Each mirror formed on a device substrate has a square or rectangular shape with a length of 4 to 15 um on each side. In this configuration, a reflected light that is not purposefully applied for an image display is inadvertently generated by reflections through the gap between adjacent mirrors, which degrades the contrast of the image display. In order to overcome such problems, the mirrors are arranged on a single semiconductor wafer substrate with a layout that minimizes the gaps between the mirrors. One mirror device is generally designed to include an appropriate number of micromirrors wherein each one is manufactured as a deflectable mirror on the substrate that displays a pixel of an image. The appropriate number of elements for a display image complies with the display resolution standard according to VESA Standard defined by Video Electronics Standards Association or television broadcast standards. The pitch between the mirrors of the mirror device is 10 μm and the diagonal length of the mirror array is about 0.6 inches when the mirror device has a plurality of mirror elements corresponding to the WXGA (resolution: 1280 by 768) defined by VESA.
Switching between dual states, as illustrated by the control circuit in FIG. 1C, positions the micromirrors in an ON or an OFF angular orientation as shown in FIG. 1A. The brightness, i.e., the gray scales of a digitally controlled image system is determined by the length of time the micromirror stays in an ON position. The length of time a micromirror is in an ON position is controlled by a multiple bit word. FIG. 1D shows the “binary time intervals” when controlling micromirrors with a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, and 8 which in turn define the relative brightness for each of the four bits, where the “1” is least significant bit (LSB) and the “8” is the most significant bit. According to the control mechanism, the minimum controllable differences between gray scales for showing different levels of brightness are represented by the “least significant bit” that maintains the micromirror at an ON position.
For example, assuming n bits of gray scales, one time frame is divided into 2b−1 equal time periods. For a 16.7-millisecond frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds.
Having established these times for each pixel of each frame, the pixel intensities are quantified such that black is 0 time period, 1 time period is the intensity level represented by the LSB, and the maximum brightness is 2^n−1 time periods. Each pixel's quantified intensity determines its ON-time during a time frame. Thus, during a time frame, each pixel with a quantified value of more than 0 is ON for the number of time periods that correspond to its intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analogous levels of light.
For controlling deflectable mirror devices, the PWM applies data to be formatted into “bit-planes”, with each bit-plane corresponding to a bit weight of the quantity of light. Thus, if the brightness of each pixel is represented by an n-bit value, each frame of data has the n-bit-planes. Then, each bit-plane has a 0 or 1 value for each display element. According to the PWM scheme as described in the preceding paragraphs, each bit-plane is separately loaded and the display elements are controlled on the basis of bit-plane values corresponding to the value of each bit within one frame. For example, the bit-plane according to the LSB of each pixel is displayed as 1 time period.
When adjacent image pixels have very coarse gray scales caused by differences in brightness, artifacts become visible between these adjacent image pixels, degrading the quality of the displayed image. The degradation of displayed image quality is especially pronounced in the bright areas of images where there are “bigger gaps” in the gray scale, i.e. brightness, between adjacent image pixels. These gaps are the result of the digitally controlled image's inability to obtain sufficient brightness levels.
The mirrors are controlled at either the ON or OFF position. Then, the brightness of a displayed image is defined by the length of time each mirror remains at the ON position. In order to increase the levels of brightness, the switching speed of the ON and OFF positions for the mirror is increased. Therefore, the digital control signals need a higher number of bits. However, when the switching speed of the mirror deflection is increased, a stronger hinge is needed to support the mirror, and to sustain the required number ON and OFF positions for the mirror deflection. Furthermore, in order to drive the mirrors' hinge toward the ON or OFF positions, the electrode requires a higher voltage. The higher voltage may be as high as thirty volts. The mirrors produced by the CMOS technology may not be suitable for such a high range of voltages, therefore requiring the use of the DMOS mirror devices. To produce the DMOS mirror and control the higher gray scale, a more complicated production process and larger device areas are required. In order to gain the benefits of a smaller image display apparatus, the accuracy of gray scales and the range of the operable voltage have to be sacrificed in conventional mirror controls.
There are many patents related to the control of quantity of light. These include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different light sources. These include U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. Also, The U.S. Pat. No. 6,746,123 has disclosed particular polarized light sources that prevent the loss of light. However, these patents or patent applications do not provide an effective solution for attaining a sufficient gray scale in the digitally controlled image display system.
Furthermore, there are many patents related to a spatial light modulation that include the U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. There are additional patented disclosures related to the image projection apparatuses. These patented disclosures include U.S. Pat. No. 5,214,420, U.S. Pat. No. 5,285,407, U.S. Pat. No. 5,589,852, U.S. Pat. No. 6,232,963, U.S. Pat. No. 6,592,227, U.S. Pat. No. 6,648,476, U.S. Pat. No. 6,819,064, U.S. Pat. No. 5,442,414, U.S. Pat. No. 6,036,318, United States Patent Application 20030147052, U.S. Pat. No. 6,746,123, U.S. Pat. No. 2,025,143, U.S. Pat. No. 2,682,010, U.S. Pat. No. 2,681,423, U.S. Pat. No. 4,087,810, U.S. Pat. No. 4,292,732, U.S. Pat. No. 4,405,209, U.S. Pat. No. 4,454,541, U.S. Pat. No. 4,592,628, U.S. Pat. No. 4,767,192, U.S. Pat. No. 4,842,396, U.S. Pat. No. 4,907,862, U.S. Pat. No. 5,214,420, U.S. Pat. No. 5,287,096, U.S. Pat. No. 5,506,597, and U.S. Pat. No. 5,489,952. However, these inventions do not provide a direct solution to overcome the above-discussed limitations and difficulties.
In view of the above problems, an invention has disclosed a method for controlling the deflection angle of the mirror to express higher gray scales of an image in US Patent Application 20050190429. According to this method, the quantity of light obtained during the oscillation period of the mirror is about 25% to 37% of the emission light intensity for a mirror that is controlled under a constant ON-state.
With this method there is no particular need to drive the mirror in high speed, making it possible to obtain a high level of gradation with a low spring constant in the spring member supporting the mirror, which allows for a reduction in drive voltage. A display image that uses the mirror device described above is broadly categorized into two types, i.e. a single-plate equipped with only one spatial light modulator and a multi-plate equipped with a plurality of spatial light modulators. In the single-plate, changing colors in turn displays a color image, i.e., the frequency or wavelength of projected light is changed by time. In the multi-plate, a color image is displayed when the spatial light modulators corresponding to different colored beams of light, i.e. frequencies or wavelengths of the light, modulate the beams of light; and are constantly combined with them.
Specifically, each micromirror device is separately controlled within one frame or one sub-frame period. For example, it is possible to control some mirrors under the ON light state for a longer period than other mirrors. This differentiates the brightness of each mirror element (i.e., the product between the intensity of the ON light and the period of the ON light state) during one frame or one sub-frame period. Separately controlling each mirror element causes each mirror to shift from the deflection angle of the ON light state to that of the OFF light state in accordance with the period in which each mirror element reflects the ON light.
Each mirror element that shifts when light is irradiated causes some mirror elements to reflect the light unstably, generating a blur in motion images. Moreover, a continuous ON position for a light source that is comprised in a projection apparatus that irradiates light onto the mirror device heats it up, and increases power consumption.